RunContext: Uninit Member

Fix an uninitialized member in `RunContext`.

Found with coverity in a downstream project.

include/internal/catch_run_context.h

@@ -128,7 +128,7 @@ namespace Catch {
         TestRunInfo m_runInfo;
         IMutableContext& m_context;
         TestCase const* m_activeTestCase = nullptr;
-        ITracker* m_testCaseTracker;
+        ITracker* m_testCaseTracker = nullptr;
         Option<AssertionResult> m_lastResult;
 
         IConfigPtr m_config;